The present invention relates to a multi-part oil ring for a piston, of the type composed of a steel band spacer and expander member which has a profiled cross section and which is transversely slotted alternately from opposite sides to cause the member to present a plurality of segments, and at least one rail which is supported by the member.
In order to reduce power losses within a piston and a piston ring group, as well as to improve their wear behavior, the automobile industry requires, inter alia, oil rings of the above-described type which are radially compressed so as to press radially outwardly on the cylinder wall with as low tangential forces as possible, on the one hand, and as uniform an oil stripping effect as possible, on the other hand.
Reduced tangential forces in the oil ring can be attained without difficulty, for example, by appropriately designing the spring characteristics of the segmented U-shaped sheet metal spacer and expander member. However, if the tangential force is reduced too much, this results in less contact pressure by the rails on the cylinder walls to such an extent that contours in the cylinder bearing surface which deviate from the ideal circular shape are compensated only to a lesser degree and oil passes the rail. This results in increased oil consumption and power loss. Compensation of this so-called form fitting capability connected with a reduced tangential force can be achieved mainly by reducing the radial rail thicknesses.
Three-part oil rings, including a U-shaped bent and segmented steel band spring spacer and expander member and two rails which rest on the arms of the U at the bottom and the top, respectively, are disclosed, for example, in U.S. Pat. Nos. 2,695,825 and 2,893,801. The ends of the U-shaped arms of the steel band spacer and expander member disclosed in U.S. Pat. No. 2,695,825 do not permit the use of rails having a small radial wall width because the resulting small radial structural depth creates the danger, particularly during installation, that the entire ring or the rails supported by the member, will slip out of the piston groove, i.e. will experience a so-called popout.
According to the concept disclosed in U.S. Pat. No. 2,893,801, the U-shaped arms of the spacer and expander member are not bent axially outwardly at their ends, but rather the bend is made approximately in the middle of each arm and the remaining free ends of the arms lie supportingly against respective sides of the ring-receiving groove during installation, so that in principle it is possible to use rails having a small radial wall width.
However, this solution is still not satisfactory because in reality the interior corners of a given piston ring holding groove are not square, as usually shown in simplified illustrations.
Rather, the corners between the sides and the base of the groove are rounded in view of fabrication requirements. It is therefore impossible for the free ends of the U-shaped spacer-expander to contact the bottom of the groove, i.e. for the purpose of centering the ring in the piston, a radial path length which would be identical with the radius of the groove bottom cannot be used to advantage.